Frames
Every WebSocket binary message carries exactly one frame: a fixed nine-byte header — type, scope, channel — followed by an opaque payload.
Above the WebSocket layer, all communication is framed. Every WebSocket binary message carries exactly one frame, and a frame never spans messages.
[type: u8][scope: u32, big-endian][channel: u32, big-endian][payload …]- Header. Nine bytes, fixed. The first byte is the frame’s type. The following eight bytes are two 32-bit unsigned integers in big-endian byte order: the scope number, then the channel number. The payload is everything after the ninth byte, running to the end of the message; there is no length field, because WebSocket already delimits messages.
- Roles. A connection has exactly two parties: one acts as the client and one as the server. The roles are fixed for the life of the connection and are independent of which party initiated it.
- Types. Seven type values are defined. Which of them a party may send depends on its role:
| Type | Client → Server | Server → Client |
|---|---|---|
0 | authorization | authorization |
1 | request | — |
2 | — | response |
3 | — | response finish |
4 | channel request | channel request |
5 | channel response | channel response |
6 | channel response finish | channel response finish |
- A frame’s kind is determined by its type and direction alone.
Where both directions define a type value, it denotes the same kind
of frame in each. Only a client sends type
1, because only a client opens a scope; only a server sends types2and3, because only a server answers one. - Undefined types are malformed. A frame bearing a type its direction does not define is malformed. The server disregards a malformed frame, and disregards a frame whose scope field names no open scope or whose channel field names no open channel where the frame’s kind reads that field. Neither occurrence ends the connection.
- Unread fields are ignored. Where a frame’s kind involves no scope or no channel, the corresponding header field is ignored: no value is prescribed for the sender to set, and the receiver does not read it.
- Nothing is acknowledged. No frame acknowledges the receipt of another. A party learns that its request arrived by being answered.
- A stream ends at its finish frame, and nowhere else. A quiet scope or channel is one that is still open, however long it has been quiet. This layer defines no timeout.
- Payloads are opaque at this layer. Their contents are defined by higher layers. This revision defines no frame type other than the seven.
The frame types each role sends are enumerated in Client Frames and Server Frames; the requirements governing scopes and channels are Scopes and Channels. The authorization frame is governed by the next layer, Authorization.